Wireless local area networks (WLAN) have become common today in homes and businesses of all sizes. A standard feature of a WLAN is the ability to multicast. Multicasting means that multiple wireless transmit/receive units (WTRUs) on the network are capable of using one transmission stream at the same time. A specific WTRU can distinguish between the packets that are addressed to it and packets meant for a different WTRU.
This is in contrast to unicast, in which there is a separate transmission stream from source to destination for each recipient. When sending large volumes of data, multicasting saves considerable bandwidth over unicasting. Therefore, the ability of multicast is an important feature, as it may improve the throughput of WLAN systems.
No matter what type of transmission scheme is used, reliability is critical. One method for achieving reliable multicasting is to have some or all recipients acknowledge the receipt of a given multicast frame. The acknowledgement can be in the form of a positive acknowledgement. That is, the recipient sends an acknowledgment that the frame has been received. Alternatively, the acknowledgement can be a negative acknowledgement, which is an acknowledgement that the frame has not been received. Lastly, the acknowledgment can be a combination of both positive and negative acknowledgements.
The IEEE 802.11 standards are a family of specifications for wireless networking that is universally accepted. At present, WLAN systems based on IEEE 802.11 standards do not support reliable multicasting. However, a joint proposal for 802.11n, the high-throughput WLAN standard, is currently under consideration by the IEEE standards body.
The joint proposal includes a power save multi-poll (PSMP) mechanism. The purpose of the PSMP mechanism is to save power in battery operated mobile WTRUs. However, this mechanism may also be used to add multicast reliability to the 802.11n standard. The PSMP mechanism allows handheld WTRUs to conserve battery power by scheduling activity on the wireless medium, rather than transmitting and/or receiving at random intervals. By reserving specific times that the WTRU is allowed to receive or transmit, the WTRU knows that it can “power down” during the non-scheduled times, as it will not be sending or receiving data
According to the Joint Proposal specification, and the Enhance Wireless Consortium (EWC) specification, PSMP is defined as a medium access control (MAC) frame that provides a time schedule to be used by the PSMP transmitter and PSMP receivers. The scheduled time begins immediately subsequent to the transmission of the PSMP frame. A downlink transmission (DLT) is defined as a period of time described by a PSMP frame that is intended to be used for the reception of frames by PSMP receivers. An uplink transmission (ULT) is defined as a period of time described by a PSMP frame that is intended to be used for the transmission of frames by a PSMP receiver.
The PSMP frame is utilized to schedule a sequence of downlink transmissions followed by uplink transmissions. Within any single PSMP sequence duration, multiple numbers of additional subsequent PSMPs (Sub-PSMPs) may be transmitted by a base station in order to allow more precise resource allocation and error recovery. An initial PSMP followed by one or more Sub-PSMPs is termed a multi-phase PSMP. A PSMP sequence (scheduled or unscheduled) may be used to transmit broadcast/multicast frames along with unicast frames. Unicast frames are scheduled after broadcast/multicast frames. Broadcast and multicast data can be transmitted using PSMP by setting the STA_ID function to a specific value, such as 0.
FIG. 1 illustrates a typical frame format of a PSMP frame 100 as defined by the 802.11 specification. The frame includes a 16 bit header 112 and multiple 64 bit sub-frames 114. Each of the 64-bit sub-frames 114 corresponds to a single WTRU. Multiple WTRUs can therefore be addressed and configured in a single PSMP frame. Each subframe 114 contains an identifier (STA_ID, 116) for each WTRU addressed by the subframe, downlink and uplink start time offsets (118,120), and downlink and uplink duration time periods (122,124). In this manner, each PSMP subframe 114, schedules, for each WTRU addressed by the subframe, the time in which the WTRU may transmit and receive data.
Another mechanism that may be used to implement multicast acknowledgments is the block acknowledge (BA) mechanism, currently used for acknowledging unicast transmissions. FIG. 2 illustrates a typical enhanced BA mechanism 200. The initiator 202 is the transmitter of the data packets 224. The initiator 202 may aggregate the data packets 224 intended for each recipient. The responder 204 is the recipient of the data packets. Once the data packets 224 are successfully received, a BA signal 200 is forwarded back to the initiator 202. Pursuant to the specification, if the BA frame 200 is sent in response to a block acknowledge request (BAR) frame, the duration/ID frame field 206 value is the value obtained from the duration/ID field of the BAR frame, minus the time required to transmit the BA 200 and its short inter-frame space (SIFS) interval 208. IF the BA frame 200 is not sent in response to a BAR the duration/ID field 206 value is greater than or equal to the time for transmission of an acknowledgement (ACK) frame plus an SIFS interval 208. The receiver address (RA) field 210 is the address of the recipient WTRU that requested the BA. The transmitter address (TA) field 212 is the address of the WTRU transmitting the BA frame.
The BA control 214 includes 12 reserved bits 216 and a terminal ID (TID) subfield 218. The BA starting sequence control field 220 is set to the same value as in the immediately previously received BAR frame.
The BA Bitmap field 122 is 64 octets in length. It is used to indicate the receiving status of up to 64 MAC service data units (MSDU's). Bit position “n”, if set to 1, acknowledges receipt of a MAC physical data unit (MPDU) with an MPDU sequence control value equal to the Block Ack starting sequence control plus the constant n. If the n bit position is set to 0, that indicates an MPDU, with an MPDU sequence control value equal to Block Ack starting sequence control plus n, has not been received. For unused fragment numbers of an MSDU, the corresponding bits in the bitmap are set to 0. As shown in FIG. 2, there is no BAR from the initiator, as it was implied by a setting in the quality of service (QoS) frame.
FIG. 3 illustrates multi-TID block acknowledgement frame format 300. The MTBA is a control frame of the BA subtype. It is used only during PSMP sequences. It consists of a MAC header 302, a BA control 304 and a 16 bit sequence 306 that is repeated for each TID. The MAC header 302 contains a 16-bit frame control field 308, a 16-bit duration/ID 310, a 48-bit RA 312 and a 48-bit TA 314. The BA control 304, is typically 16 bits. It consists of a single bit ACK policy frame 316, a single bit MTID frame 318, a single bit compressed BA frame 320, 10 reserved bits 322 and a 3-bit number of TID frame 324. Repeated for each TID is a 16-bit BA control per TID frame 326, a 16 bit BA starting sequence control frame 330, a 32 bit BA bitmap 330 and a 32 bit Frame Checksum (FCS) 332. A MTBA 300 allows for single frame to respond to all BARs for multiple TID. It is used only within PSMP sequences instead of BA.
FIG. 4 illustrates a typical PSMP sequence 400. The PSMP frame 402 specifies the time allotted for DLT1 404, DLT2 406 and ULT1 408 and ULT2 410. The frame is transmitted prior to every DLT/ULT sequence. As shown in FIG. 4, the PSMP frame 402 is transmitted first, then DLT1 404 is transmitted for the time allotted in the PSMP frame. The ULT1 408 is transmitted in the time allowed by the PSMP frame, and the sequence repeats.
FIG. 5 illustrates an example of a PSMP resource request 500 and FIG. 6 illustrates an example of a PSMP retransmission 600. Referring to FIG. 5, the PSMP sequence allows the base station 502 (designated as access point (AP)) to create effective service periods for scheduled automatic power save delivery (S-APSD). The PSMP sequence provides benefits such as statistical multiplexing of retries, activity cycles and rate variations. In voice over internet protocol (VoIP) applications, the benefit is up to twice the throughput, resulting from sharing an allocation for retries within the current aggregate service point.
Some recent proposals to the standards committees have included new rules for PSMP operation. Some of these rules pertain to access point or base station operation. As an example, rules have been proposed that the specification only includes stations (WTRUs) within PSMP if the WTRU is capable, as advertised in the WTRU's high throughput (HT) capabilities. Also, the base station should obey rules regarding minimum times between DLT and ULT. Also, the base station may set bits in “TID set” field to provide recommendations to the WTRU for use of ULT. Additionally, the base station should use end of service point (EOSP) bits to signal the end of data delivery to a WTRU. The base station gives the WTRU permission to return to sleep in order to conserve power.
Other proposed rules include that the ACK policy setting in DLT frames is “PSMP/MTBA”. Other proposed rules include rule regarding Service Interval Granularity to include SIG advertised by the Base Station to allow the WTRU to determine appropriate Traffic Specification (TSPEC) service interval request to match Base Station PSMP service intervals. Also, the PSMP may be used in context of unscheduled (U)-APSD or scheduled (S)-APSD.